This invention relates generally to thin-film semiconductor materials. Within the context of this disclosure, thin-film semiconductor materials comprise materials which are deposited by building up thin layers on a substrate, typically through a vapor deposition process. Such processes include plasma deposition processes (also referred to as plasma chemical vapor deposition processes), wherein a process gas typically comprised of a semiconductor precursor and a diluent gas is subjected to an electrical field which ionizes the gas so as to create a reactive plasma which decomposes at least some of the components of the process gas and deposits a layer of semiconductor material onto a substrate maintained in, or in close proximity to, the plasma. Non-plasma deposition processes such as non-plasma chemical vapor deposition and evaporation processes may be similarly employed for the preparation of thin-film semiconductor materials.
Thin-film semiconductor materials are generally considered to be disordered semiconductor materials insofar as they are lacking in long-range order and are not single crystalline or polycrystalline materials. Thin-film semiconductor materials may be amorphous materials which manifest only local or intermediate range ordering (although they may include, at times, regions of higher ordering). Thin-film materials also include microcrystalline materials, which are distinguishable from long range ordered materials such as single crystalline materials and polycrystalline materials as well as from other thin-film materials such as amorphous materials.
U.S. Pat. No. 4,600,801 discloses a highly conductive, highly transparent P doped, microcrystalline semiconductor alloy material having a particular utility in N-I-P type photovoltaic devices, and the disclosure thereof is incorporated herein by reference. As specifically disclosed therein, microcrystalline materials are distinguishable from amorphous materials insofar as they exhibit a threshold volume fraction of crystalline inclusions at which substantial changes in key parameters, including electrical conductivity, band gap and absorption constant occur.
The concept of microcrystalline materials exhibiting a threshold volume fraction of crystalline inclusions at which substantial changes in key parameters occur can best be understood with reference to the percolation model of disordered materials. Percolation theory, as applied to microcrystalline materials, analogizes properties such as the electrical conductivity manifested by microcrystalline materials, to the percolation of a fluid through a non-homogeneous, semi-permeable medium such as a gravel bed. Microcrystalline materials are formed of a random network which includes low conductivity, highly disordered regions of materials surrounding randomized, highly ordered crystalline inclusions having high electrical conductivity. Once these crystalline inclusions attain a critical volume fraction of the network (which critical volume will depend, inter alia, upon the size and/or shape and and/or orientation of the inclusions), it becomes a statistical probability that said inclusions are sufficiently interconnected so as to provide a low resistance current path through the network. Therefore, at this critical or threshold volume fraction, the material exhibits a sudden increase in conductivity. This analysis (as described in general terms relative to electrical conductivity herein) is well known to those skilled in solid-state theory and may be similarly applied to describe additional changes in physical properties of microcrystalline materials such as optical gap, absorption constant, etc.
The onset of this critical threshold value for the substantial change in physical properties of microcrystalline materials will depend upon the size, shape and orientation of the particular crystalline inclusions, but is relatively constant for different types of materials. The shape of the crystalline inclusions is critical to the volume fraction necessary to reach the threshold value. There exist one-dimensional, two-dimensional and three-dimensional models which predict the volume fraction of inclusions necessary to reach the threshold value, these models being dependent on the shape of the crystalline inclusions. For instance, in a one-dimensional model (which may be analogized to the flow of charge carriers through a thin wire), the volume fraction of inclusions in the amorphous network must be 100% to reach the threshold value. In the two-dimensional model (which may be viewed as substantially conically shaped inclusions extending through the thickness of the amorphous network), the volume fraction of inclusions in the amorphous network must be about 45% to reach the threshold value. Finally, in the three-dimensional model (which may be viewed as substantially spherical shaped inclusions in a sea of amorphous material), the volume fraction of inclusions need only be about 16-19% to reach the threshold value. Therefore, amorphous materials may incorporate crystalline inclusions without being microcrystalline as the term is defined herein. Likewise, microcrystalline materials may include amorphous regions, consistent with the definition herein.
Microcrystalline semiconductor materials generally have higher electrical conductivities and better stabilities then do corresponding amorphous semiconductor materials. As a consequence, microcrystalline semiconductor materials are finding increasing utility in particular semiconductor applications. For example, in the field of photovoltaics, microcrystalline semiconductor layers are used either alone, or in combination with amorphous semiconductor layers to fabricate a variety of photovoltaic device configurations. For example, U.S. Pat. No. 4,600,801 referred to above discloses P-I-N type photovoltaic devices in which the P layer thereof is fabricated from a microcrystalline alloy of silicon hydrogen and fluorine. A paper entitled: “Material and Solar Cell Research in Microcrystalline Silicon”, Shah et al., Solar Energy Materials and Solar Cells 78 (2003) 469-491, discloses photovoltaic devices made entirely out of microcrystalline alloys of silicon and hydrogen. U.S. Pat. No. 6,472,248 discloses photovoltaic devices which are comprised of amorphous semiconductor material and stacked microcrystalline layers of semiconductor material having differing morphologies.
As is known in the prior art, and as is recognized in the '248 patent, microcrystalline silicon and its alloys can exist in various morphologies. For example, the material may comprise spherical crystallites in a substantially amorphous matrix; it may comprise more elongated crystals in a matrix; or, it may comprise a columnar structure comprised of relatively long crystals oriented approximately normal to a substrate. The definition of microcrystalline material given above acknowledges and encompasses all of such morphologies.
Plasma deposition processes of the type described above can be implemented under conditions which favor the deposition of amorphous or microcrystalline materials and such deposition conditions are disclosed, for example, in the above-referenced U.S. Pat. No. 4,600,801 which is incorporated herein by reference. It is to be understood that plasma deposition processes may be carried out using a very wide range of electromagnetic energy, including frequencies ranging from audio frequency to radio frequency to very high frequency and up through microwave frequencies; and the present invention can be utilized with all of such frequencies.
The prior art has recognized that the optimum microcrystalline silicon alloy layers for photovoltaic devices are deposited under deposition conditions which are close to the amorphous/microcrystalline threshold. In this regard see, for example, Shah et al. op cit. Shah has likewise recognized that microcrystalline silicon having a columnar or other large grain structure is generally undesirable for the fabrication of photovoltaic devices and has stated that, in a plasma deposition process, the use of process gases which have high levels of hydrogen dilution will cause the growth of large grains. The prior art also recognized that a plasma deposited amorphous semiconductor material will tend to become more ordered as its thickness increases. This teaching is found in U.S. Pat. No. 6,274,461, the disclosure of which is incorporated herein by reference.
In the course of preparing photovoltaic cells from microcrystalline materials, the inventors hereof found that cell performance, as measured by one or both of open circuit voltage (Voc) and fill factor (FF), decreases rapidly as thickness of the device was increased. The rate of decrease in performance as a function of thickness was too great to be attributable to the increased distance through which charge carriers had to travel, which led the inventors hereof to surmise that the material quality of the deposited microcrystalline semiconductor must be deteriorating as its thickness increases.
Table 1 summarizes data from a series of experiments in which six N-I-P type photovoltaic devices were prepared by a plasma activated glow discharge deposition process carried out utilizing very high frequency energy of 70 MHz. Each cell comprised a body of intrinsic microcrystalline silicon-hydrogen alloy material interposed between relatively thin P and N doped layers of microcrystalline silicon-hydrogen alloy material. As will be seen from Table 1, the thickness of the intrinsic layer varied from 335 nm in sample 1 to 1980 nm in sample 6.
Performance parameters for each of the cells were measured under AM-1.5 illumination. These parameters include the figure of merit Q measured in terms of in A/cm2, open circuit voltage (Voc), fill factor (FF) and maximum power (Pmax) measured in terms of mW/cm2. Fill factor is a good measure of material quality of a semiconductor material used in a photovoltaic device; and as will be seen (disregarding the relatively thin cell of sample 1), fill factor decreases as the thickness of the intrinsic layer increases. Likewise, open circuit voltage of the cell also drops as the intrinsic layer becomes thicker, and this suggests that the grain size of the material forming the intrinsic layer is increasing as the layer thickness increases. While not wishing to be bound by speculation, the inventors hereof have postulated that the degree of ordering of the microcrystalline semiconductor material is increasing as the thickness of the deposit increases. This leads to the formation of undesirable large-size grains of semiconductor material.
TABLE 1SampleThicknessQVocPmaxNo.(nm)(mA/cm2)(V)FF(mW/cm2)13359.450.470.6512.89247010.980.4660.6723.44372012.990.4390.643.654104014.80.4340.6213.995130516.510.4140.5783.956198017.870.3930.5103.58
Having identified this problem, the inventors hereof recognize that there is a need for some method or means for moderating the grain size of microcrystalline semiconductor materials so as to prevent the undesirable growth of large grains in the plasma deposition of microcrystalline semiconductor materials.
Owing to the fact that microcrystalline group IV semiconductor materials have an indirect band gap, their optical absorption coefficients are much lower than those of corresponding amorphous semiconductor materials. Hence, microcrystalline semiconductor layers incorporated in photovoltaic cells, electrophotographic receptors and other photoresponsive devices must be made much thicker than corresponding amorphous layers used in analogous devices. For this reason, the fact that the material quality of such microcrystalline semiconductor layers decreases with increasing thickness is a very serious limitation on the use of such layers, and there is a significant need for preventing this decrease in material quality.
As will be explained in greater detail hereinbelow, the present invention recognizes that by profiling the dilution of a process gas employed for the plasma deposition of a semiconductor material, the morphology of a microcrystalline layer may be advantageously controlled.